Constructs have been designed for some time for implantation in the spine which generally include a series of bone fasteners, such as hooks or screws, that are secured to the vertebrae, and which are used to hold stabilizer means such as a rod or plate that spans several vertebrae for stabilization, fixation, and/or for alignment of the vertebrae.
Typically, a spinal rod assembly includes two sets of rods that are fixed to adjacent vertebrae on either side of the spinous process to span a section of spine. The bone anchors may include a number of fixation means, such as screws or hooks, that are used for fixation to the spine, and anchor means, such as rod anchors that includes means to secure the rod to the fixation means. In some systems these component parts are a single integral unit, while other systems utilize a number of assembled components such as for the anchor portion of the assembly.
There are a number of considerations which go into the design of the assembly. The constructs need to be relatively easy to assemble, to be safe for the area of implantation, to provide for flexibility of use to accommodate a number of different indications for implantation and for variations in individuals who require their use, to be strong, yet minimally invasive and low profile, to be useful for manipulation, as well as for the maintenance of the desired alignment of the spine.
There are spinal systems which have successfully worked out these design criteria for the lumbar and the thoracic areas of the spine. However, the occipito-cervical area is deserving of even closer scrutiny as the areas for implantation become smaller and the associated nerves and arteries need to be identified and avoided. Moreover, there is less muscle mass to cover the implant.